1. Field of the Invention
The present invention relates to a face detection device that detects the position and the size of one or more face images included in an image.
2. Description of the Related Art
In recent years, not only digital cameras but also recording media having large capacity (e.g. hard disk drives) are widely used. The amount of movies, each of which is a set of still images, and/or still images recorded thereon by a person has become huge.
Mobile devices associated with cameras (e.g. personal digital assistants and cell phones) are also widely used, and many persons use the devices to send e-mails to which one or more image files are attached. The persons detect person images from a group of images photographed by the devices to attach the one or more detected images to the e-mails. Thereby, the persons often satisfy their identity, and add their feelings and/or emotion to the e-mails.
This situation requires technology for searching, detecting, tagging and recommending only one or more desired face images among the huge amount of still images. Herein, a “desired” image is a face image that suits some purposes, which may be an image that can specify a specific person, a well-taken face image that can be attached to the e-mails, or a face image that is good to be edited.
The composition of the images is various, which may be not only the snapshot composition of an upper half of the body used for personal ID cards, but also that of a profile face, or that of a set of small face images in souvenir photographs. Each of frame images constituting the movies is the same as above.
With respect to the content of the images, needless to say, a case where no person image is included (e.g. a landscape image), in some cases, a person image may be intentionally considered as not an image of a photographic subject but an image of a part of landscape. This assumption requires dealing with face directions and face rotation.
Document 1 (U.S. patent publication No. 2002/0159627) discloses face detection technology that deals with various directions. A judgment unit that judges whether one or more face images exist in a partial rectangle area of an image is provided. When an area where one or more face images exist therein should be detected, the judgment unit repeats the processes of defining the partial rectangle area; and raster-scanning the defined partial rectangle area from a corner of the image, thereby judging whether one or more face images exist in the defined partial rectangle area. The partial rectangle area size is defined based on the size of a face image to be judged.
In general, various size face images (from a big face image to a small face image) may exist in the image. Accordingly, it is required to repeat judging whether or not the partial rectangle area is an area including one or more face images, while changing the size of the partial rectangle area from a small size to a big size.
The face judgment unit scales the partial rectangle area into a predetermined size, and judges whether the area includes a face image. The face judgment unit, beforehand, has already learned a group of face images in various directions (e.g. frontal face, profile, and so on.) utilizing a statistical learning means (e.g. neural network), which performs face judgment utilizing the learning result.
To be more specific, a learning result concerning frontal face images and a learning result concerning profile face images have been separately stored. The face judgment unit performs face judgment utilizing these learning results.
Document 2 (Published Japanese Patent Application Laid-Open No. 2001-357404) discloses a method for detecting a desired image from an image database. A direction of a photographic subject in an image of the image database is detected, and then an image including the photographic subject looking toward a predetermined direction is detected from the image database based on a result of the detection.
However, according to document 1, a rate of judging areas including no face image as face areas (in other words, an false positive rate) is high. In particular, in the case of a profile image (when one of two eyes may not be included therein), the false positive rate becomes higher than that of a frontal face image.
Once false positive occurs, a landscape image may be detected when the desired image should be detected from the image database, and this function may become useless and unreliable. In order to reduce false positive, it is also considered to raise the criterion of judgment (e.g. threshold) for judging whether one or more face images exist. This, however, causes judging that an image includes no face image when the image should be judged to include one or more face images.
As described above, since the false positive rate of the profile image is higher than that of frontal face image, raising a threshold for the profile image to reduce false positive causes failing to correctly judge one or more images that should be judged to include a face image, thereby spoiling the original function of face image-detecting.
In document 2, a predetermined direction has been defined, and then it is judged whether or not an inputted image relates to the predetermined direction. Afterward, an image relating to the predetermined direction is detected. The predetermined direction, however, is only one. Since a face may practically look toward various directions, it is hard to say that this method is suitable. Furthermore, according to document 2, since the false positive rate of the profile image is high, and incorrect direction may often be detected, which is inadequate.